Phosphatases or phosphomonoesterases are the hydrolytic enzymes that cleave the ester bond between the phosphate group and the organic residue of the organic phosphates. Based on the optimum pH for the activity, phosphatases are of two kinds: acid and alkaline. Acid phosphatases show maximum activity at acidic pH around 6 whereas alkaline phosphatases show maximum activity at alkaline pH around 11.
The p-nitrophenyl phosphate acts as a substrate for both acid and alkaline phosphatases. On hydrolysis by either of the enzymes, it yields p-nitrophenol, the color of which is read spectrophotometrically.